Benzoic acid can be prepared by a process such as oxidation of toluene. For example, there is a report that the liquid phase oxidation of toluene gives benzoic acid in a yield of 40% or so. Benzoic acid obtained by a process such as mentioned above contains a small amount of impurities even after removal of low-boiling substances such as toluene beforehand. The impurities here include benzaldehyde, benzyl alcohol, benzoic acid esters, biphenyls, p-phenylbenzoic acid, phthalic anhydride and toluic acid and, as they cause problems relating to emission of odor and coloration, they should desirably be removed from the product benzoic acid.
The following processes are known for the purification of benzoic acid: treatment with sulfuric acid followed by rectification as disclosed in Japan Kokoku Tokkyo Koho Sho 61-25013 (1986); vaporization in an inert gas followed by crystallization under specified conditions as disclosed in Japan Kokoku Tokkyo Koho Sho 57-11899 (1982); distillation in the presence of aliphatic amines as disclosed in Japan Kokoku Tokkyo Koho Hei 6-43367 (1994); and purification by supercritical extraction as disclosed in Japan Kokai Tokkyo Koho Sho 61-130257 (1986). None of these processes, however, satisfied all the factors involved in purification such as efficiency, equipment, and cost.
A process based on crystallization such as recrystallization is also known for the purification of crystalline solids, but this too faces the aforementioned problems and is not well suited for application on a commercial scale.
An object of this invention is to provide a process for purifying benzoic acid which permits of continuous preparation of high-purity benzoic acid in a relatively simple equipment at high efficiency.